Aqua Regia

Aqua regia is the result of mixing a solution of hydrochloric acid (HCl) and a solution of aqua fortis or more in the know with Nitric Acid (HNO3), which is part of an oxidant the most powerful.

Aqua regia is a highly corrosive, fuming yellow or red solution.  In a mixing between Nitric Acid (HNO3) and hydrochloric acid (HCl) is 3: 1, That is the volume of a solution of 3 parts hydrochloric acid (HCl) was added to 1 volume part of the solution of Nitric Acid (HNO3) .

Aqua regia or in Latin is more known as royal water is a chemical solvent all the metals. It is one of the few reagents that dissolves as gold, platinum and palladium from substrates, particularly in microfabrications and microelectronics labs.

It was so named because it can dissolve the so-called royal, or noble metals, although tantalum, iridium, and a few other metals are able to withstand it. Glassware may also be washed with aqua regia to remove organic compounds only in trace amounts. Aqua regia solutions are extremely corrosive and may result in explosion or skin burns if not handled with extreme caution.

*  Decomposition Of Aqua Regia

Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.

HNO3 (aq) + 3 HCl (aq) → NOCl (g) + Cl2 (g) + 2 H2O (l)

Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.

2 NOCl (g) → 2 NO (g) + Cl2 (g

      Aqua regia is used in etching and in certain analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. In practice Aqua regia is often used as a solvent in  the processing of gold metal, and chemicals are often used as a gold refining.

Aqua regia dissolves gold, even though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3 ).

The hydrochloric acid provides a ready supply of chloride ions (Cl-), which react with the gold to produce chloraurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl4-).

This results in a removal of gold ions from solution and allows further oxidation of gold to take place, and so the gold is dissolved. In addition, gold may be oxidized by the free chlorine present in aqua regia

* The process is very simple dissolution happens when metal reacts gold by aqua regia solution.

Chemical Reactions Aqua Regia Gold

Au + 3HNO3 + 4HCl → HAuCl4 + 3NO2 + 3H2 O

       Au (s) + 3 NO3-(aq) + 6 H + (aq) → Au3 + (aq) + 3 NO2 (g) + 3 H2O (l) and

Au3 + (aq) + 4 Cl-(aq) → AuCl4-(aq).

 
 After the reaction is complete then the next step is the process of settling and separation of the deposition solution. The next solution in neutral with the addition of urea to reduce the residual HNO3 As happened in the following chemical reaction

6 HNO3 + 5CO (NH2) 2 = 8N2 + 5CO2 + 13H2O

And gold gained by cementation, ie precipitation with the addition of sodium metabisulfide, as in the following reaction:

2HAuCl 4 + 2NaHSO3 = 2Au + 4HCl + Na2 SO4 + SO2

or with the addition of hydrogen peroxide (H2O2) and Sodium Hydroxide (NaOH). Further sediment left in the wash with hot water and then dried in the melt to be made in a metal

* The process of dissolution of metallic platinum with aqua regia solution

 

Similar equations can be written for platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product.

Chemical Reactions Aqua Regia Platinum

Pt (s) + 4 NO 3- (aq) + 8 H+ (aq) → Pt4+ (aq) + 4 NO2 (g) + 4 H2O (l)
3Pt (s) + 4 NO 3- (aq) + 16 H+ (aq) → 3Pt4+ (aq) + 4 NO (g) + 8 H2O (l)

The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion.

 Pt4+ (aq) + 6 Cl- (aq) → PtCl62- (aq)

Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H2PtCl4) and nitrosoplatinic chloride ((NO)2PtCl4). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed.

Pt (s) + 2 HNO3 (aq) + 4 HCl (aq) → (NO)2PtCl4 (s) + 3 H2O (l) + 1/2 O2 (g)
(NO)2PtCl4 (s) + 2 HCl (aq) → H2PtCl4 (aq) + NOCl (g)

The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with chlorine while heating.

H2PtCl4 (aq) + Cl2 (g) → H2PtCl6 (aq)